Fewer circumcisions and more venereal disease may help explain why AIDS has spread widely through heterosexual intercourse in parts of Africa but not in the United States, a report published this week concludes.

The study, conducted in Kenya, found that men with genital sores are seven times more likely than usual to catch the AIDS virus. Being uncircumcised nearly triples the risk.The researchers say their work suggests that the high prevalence of venereal diseases that cause genital sores, with more prostitution and fewer circumcisions, help explain why AIDS has been spread through sex between men and women in parts of central Africa.

In the United States, AIDS is largely transmitted through homosexual intercourse and intravenous drug use. But in Africa, experts believe, AIDS is primarily a heterosexual disease.

The study was conducted with 340 men who caught sexually transmitted diseases from prostitutes in a Nairobi neighborhood where more than 85 percent of the street walkers were infected with HIV, the virus that causes acquired immune deficiency syndrome.

"Clearly, prostitute contact, genital ulcer disease and an intact foreskin are all risk factors for HIV infection," said Dr. J. Neil Simonsen, who directed the study published in the New England Journal of Medicine.

The researchers found a strong link between AIDS risk and a sexually transmitted ailment called chancroid, a bacterial disease common in Africa but relatively rare in the United States. However, Simonsen said that two common American diseases, genital herpes and syphilis, also probably increase the chance of AIDS.

The report said that controlling diseases that cause genital sores could be an important means of stopping the spread of AIDS.

The researchers speculate that the sores might raise AIDS susceptibility by causing breaks in the skin where the virus can enter. People with genital sores also may be more likely to pass on the AIDS virus if they are already infected with it.

The researchers are unsure how being uncircumcised might increased the AIDS risk. But Simonsen said it's possible that "the presence of a foreskin offers a nice, moist micro-environment for the AIDS virus to survive longer and have a greater chance of penetration through little nicks in the skin that are a normal part of sexual intercourse."

Even though the report found an association between circumcision and AIDS risk, it does not prove that circumcision actually helps protect people from infection.

"Based on this data, it's not possible to recommend that everybody run out and get circumcised," Simonsen said. "The association is strong and probably will be causal, but you can't conclude that."

The researchers said that 85 percent of white American men are circumcised. Circumcision practices vary from tribe to tribe in Africa. In the Kenyan study, three-quarters of the men were circumcised.

Simonsen, a physician at Case Western Reserve University in Cleveland, conducted the work with researchers from the University of Nairobi, the Kenya Medical Research Institute, the Nairobi Special Treatment Clinic, the University of Manitoba and the Institute of Tropical Medicine in Belgium.

In an accompanying editorial, Dr. Jonathan M. Mann of the World Health Organization estimated that there have been 250,000 cases of AIDS worldwide, and between 5 million and 10 million people are probably infected with the AIDS virus.

He said other factors that may contribute to AIDS in Africa are blood transfusions, the use of unclean needles and infection of babies by their mothers during childbirth.